Also run CI on Java 21

Author: davidcooke2

.github/workflows/gradle-build.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        java: [ 11, 17 ]
+        java: [ 11, 17, 21 ]
     name: Java ${{ matrix.java }}
     steps:
       - uses: actions/checkout@v2

.github/workflows/jcstress-manual.yml

@@ -13,7 +13,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        java: [ 11, 17 ]
+        java: [ 11, 17, 21 ]
     name: Java ${{ matrix.java }}
     steps:
       - uses: actions/checkout@v2

.github/workflows/jcstress-quick.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        java: [ 11, 17 ]
+        java: [ 11, 17, 21 ]
     name: Java ${{ matrix.java }}
     steps:
       - uses: actions/checkout@v2